The present invention relates to decorative plates which are used as building materials and interior use materials, and more particularly to a method for producing thermoset-resin surfaced decorative plates such as a melamine surfaced decorative plate, a polyester surfaced decorative plate, a DAP surfaced decorative plate, an epoxy surfaced decorative plate and a guanamine surfaced decorative plate.
Thermoset-resin surfaced decorative plates are suitable for use as wall- materials, flooring materials, and materials for household furniture, kitchen fixtures and the like, which are required to have high strength and high resistance to heat. Decorative plates of this type are, in general, produced by printing a pattern on a porous base sheet by means of gravure printing, offset printing or the like, impregnating the base sheet with a thermosetting resin, and hot-pressing the resulting base sheet to harden the thermosetting resin.
Titanium-containing paper has been used as the porous base sheet of the above conventional decorative plates. In order to ensure thorough penetration of a resin into the base sheet, the gas permeability of the base sheet is made low, and a tangle of pulp fiber which composes the base sheet is made rough. Moreover, a large amount of titanium white or a coloring pigment is employed when the paper (base sheet) is made. However, such a porous base sheet is inferior in the printability to a paper such as coated paper or art paper. A pattern therefore cannot be printed on the base sheet with sufficiently high reproducibility, and numerous fine non-printed portions are produced in a pattern-printed area. Furthermore, an ink cannot be stably transferred to the base sheet, so that it is difficult to uniformly control the quality of the product. Printing methods of the above type are disclosed in Japanese Patent Publications Nos. 4540/1951 and 7737/1959.
In the case where a pattern cannot be easily printed on a base sheet, a transfer printing technique may be utilized. According to this technique, a pattern layer is transferred to a base sheet with the aid of an adhesive layer. For instance, Japanese Patent Publication No. 22732/1963 and Japanese Laid-Open Patent Publication No. 148662/1984 disclose a method for producing decorative plates utilizing the transfer printing technique. In this method, a transfer printing sheet containing a grain-pattern layer is superposed on a base sheet which has been impregnated with a resin in advance, the transfer printing sheet and the base sheet are then hot-pressed, and the transfer printing sheet is finally peeled off from the base sheet to give a decorative plate having the wood-grain pattern.
However, the above method for producing decorative plates utilizing the transfer printing technique is not a perfect one. One of the reasons is that the pattern layer cannot firmly be adhered to the base sheet impregnated with a resin when the pattern layer (a wood-grain pattern) is transferred to the base sheet. In the case where a base sheet to which the pattern layer can firmly adhere is employed, the resin cannot be thoroughly penetrated into the base sheet. More specifically, in the above-described conventional method, after a porous base sheet is impregnated with a resin, a transfer printing sheet is superposed on the base sheet. The transfer printing sheet and the base sheet are then hot-pressed so that the resin contained in the base sheet can be penetrated into the pattern layer. The base sheet and the pattern layer are thus joined by the resin into one. However, we have found that when a thick and dense adhesive layer is used in order to firmly adhere the pattern layer to the surface of the base sheet, the resin cannot be uniformly penetrated into the pattern layer in the above joining process. As a result, the yield of the product is lowered. On the other hand, when a thin and coarse adhesive layer is used to ensure thorough penetration of the resin into the pattern layer, the interfacial adhesion between the pattern layer and the base sheet is decreased. This also brings about deterioration of the quality of decorative plates.
Another problem is that after the pattern layer is transferred onto the base sheet by hot pressing, a substrate of the transfer printing sheet is not always finely separated from the pattern layer. This is because the substrate of the transfer printing sheet and the pattern layer tend to be firmly adhered to each other by the resin penetrated into the pattern layer.
Thermoset-resin surfaced decorative plates require that fiber which composes a base sheet, an adhesive layer and a pattern layer are firmly adhered and joined by an impregnating resin. If not, blisters are formed, or separation between the layers is caused when heat is applied to the decorative plate, for example, when a pan, a pot or the like containing hot water is placed on the decorative plate. Blisters are also formed on such a decorative plate when it is subjected to post-forming in which it is bent with the application of heat.